The
Biggest Bang of Them All

When did time begin?
Where did we come from? What is our destiny? Three very fundamental questions
that have always been on human minds.

What was there
before the beginning of time? What is there beyond the edge of the universe?
Is it possible to travel back in time? Three more bewildering questions that
are bound to fascinate youngsters and stump teachers.

Both of us remember
asking ourselves these kinds of questions as kids. Unfortunately, nobody around
either of us seemed to have the answers. Yet we had to know! We should become
astronomers and figure it all out, we thought.

Well, we found
out it's not that easy. Scientists have made significant progress in recent
years, but plenty of issues remain controversial. This is often illustrated
by newspaper headlines: "Scientist sees face of God, confirms Big Bang model"
(a reference to results from the Cosmic Background Explorer satellite), "Is
the cosmos younger than some of the stars it contains? Cosmology is in chaos"
(attempts to measure the expansion rate of the universe), "Astronomers discover
billions of new galaxies they did not expect" (Hubble Space Telescope observations),
"Physicists describe the grim end of the world" (speculations on the fate of
the universe).

All these issues,
and many more, make cosmology an exciting field. By definition, cosmology is
the study of the universe as a whole, its history, and its overall contents.
Cosmology attempts to answer some of the most basic questions we have about
the reality we inhabit.

The
Lone Universe

Cosmologists are in
a unique situation among scientists. There is only one specimen for us to study:
the one and only universe we live in. We cannot reproduce it; we cannot take another
one and see what happens to it; we cannot compare it to anything else, as astronomers
routinely do with planets, stars, and galaxies. Nor can we experiment with what
we are studying. We can only observe it passively.

Moreover, we are
inside the universe (by definition). That makes it even more difficult to study.
Cosmologists are like dentists trying to operate on their own teeth. We are
trapped inside the object of our study. In science, it always helps to look
at things from the outside. Zoologists try not to be seen by the animals they
study, in order not to influence their behavior. Meteorologists look at storm
systems from above using weather satellites. But we cannot escape from the universe.
We cannot go outside it and take a look.

This means that
we have to be careful about the meaning we give to what we see nearby and far
away. If you look around you, you can probably see a wall, a door, a window,
a closet. If you look farther away, you can see a street, other buildings, maybe
mountains in the distance. In between could be a farm, a lake, a forest, a highway.
So you can see very different things in different directions and at different
distances.

If you could keep
looking farther away, you would eventually see the ocean. At that point, you
could talk in terms of land vs. water: here a continent, there an ocean, another
continent, and so on. Someone else could do the same from anywhere on Earth,
and while his or her immediate neighborhood could be quite different from yours,
on a large enough scale ­ that of the whole Earth ­ the two of you
would have the same basic description: land vs. water.

In cosmology,
galaxies are the land and the space between galaxies is the water. It is only
on the scales of galaxies and larger groupings that we can talk about the universe
in general. On that scale, your exact position in the universe does not matter,
because it looks roughly the same from any location in any direction: galaxies
all.

This observation
is the starting point of cosmology. Just as Earth is not the center of the solar
system, it is not the center of the universe, but rather some random location
equivalent to any other location. In technical terms, cosmologists say the universe
is "homogeneous" and "isotropic" on its largest scales. Homogeneous means that
the universe has the same basic composition and structure everywhere. Isotropic
means that it looks basically the same in every direction.

Cosmologists also
expect that the same laws of physics ­ gravity, motion, electricity and
magnetism, and so on ­ apply everywhere at all times. This, too, is supported
by observations. If the laws varied even slightly, distant stars would refuse
to shine, orbits would go haywire, light would look sickly. We see none of this.
The remarkable uniformity of the universe is what allows us to study it in its
entirety.

How
big is the universe, anyway? Each of these bubbles is about 300 times
as wide as the previous one. If you start with your hometown and zoom
out nine times, you reach the edge of the observable universe. This diagram
is a simple example of a logarithmic scale. Incidentally, if you went
the other direction, shrinking each bubble by a factor of 300, you would
end up with a speck smaller than the smallest known subatomic particle.
So, human beings are at the middle of the range of cosmic scales. We are
about as small in comparison to the universe as subatomic particles are
in comparison to us. Diagram by Kathleen L. Blakeslee for the ASP.